In making ceramic-based electronic packages for housing integrated circuits and in the production of other ceramic products, it is often desirable to have the option of using a reducing atmosphere for thermal processing of the ceramic material(s). In some circumstances, the use of a reducing atmosphere may be necessary to avoid unwanted oxidation (of the ceramic and/or any co-fired metals) and to obtain a useful product.
In the manufacture of many ceramic products, such as ceramic-based electronic packages, the ceramic powder is combined with a binder and formed into a green ceramic tape. Metals may optionally be placed on the tape surface and/or at locations in the tape by known metallization techniques. A series of the green tapes are then laminated and fired to produce a densified ceramic product (e.g. a dense ceramic base for use in an electronic package.)
Green ceramic tapes can be formed in a variety of ways such as by tape casting or by extrusion. Tape casting is the most widely used and generally preferred technique from an economic standpoint.
When green ceramic tapes are used to form dense ceramic products, the binder in the green tape is typically removed during thermal processing of the tape laminate. While complete removal of the binder is generally preferred (especially for ceramics used in electronic packaging), often residual material such as components and/or reaction products from the binder can remain in the ceramic. For organic binders, a typical residual material is elemental carbon. Residual elemental carbon is generally a less prevalent problem for ceramics that are processed in oxidizing atmospheres. Oxygen can react with the binder and/or residual carbon to form gaseous materials that can escape from the green tape laminate.
For ceramics processed in reducing atmospheres, the problem of residual materials (e.g. carbon) is more difficult to overcome Generally, effective binder removal can only be accomplished by use of some sort of binder removal catalyst, by use of a prolonged low temperature heating schedule, by use of wet gas atmospheres (e.g. U.S. Pat. No. 4,234,367) or by use of a special binder.
Catalysts add expense to the process and may also leave undesirable trace materials (especially if the ceramic is to be used in electronic packaging.) Prolonged heating schedules add significantly to the cost of production and may only be marginally effective. Wet gases, even when they contain hydrogen, may produce undesired oxidation and/or may require elaborate control of heating and atmosphere conditions. While special binder systems have been used to overcome this problem (e.g. polyethylene used in U.S. Pat. No. 4,920,640), such special binders are generally not amenable to formation of green tapes by tape casting.
Accordingly, there remains a need for green tape formulations which are amenable to tape casting and which can be effectively removed from a green ceramic laminate during thermal processing in reducing atmospheres without the use of catalysts, prolonged heating schedules or wet gases.